1. Field of the Invention
This invention relates to an electromagnetic trigger device in a camera capable of effecting time photography.
2. Description of the Prior Art
In cameras such as, for example, cinecameras wherein a motor is driven by operation of an electromagnetic trigger device for controlling the stop position of shutter and a mechanism for transporting a film and actuating a shutter mechanism is driven by the motor to effect photography, decreasing the exposure time during photography by decreasing the opening angle of the shutter may be done independently of the number of revolutions of the motor, whereas increasing the exposure time by increasing the opening angle of the shutter encounters structural limitations and inevitably depends on the number of revolutions of the motor. For a long time exposure, therefore, the number of revolutions of the motor must be reduced, whereas stable rotation of the motor at low speeds suffers from much limitation and the use of gear reduction would unavoidably involve complication thereof. Also, rotation of the motor continues throughout exposure operation and this leads to great power consumption, which in turn offers problems to ordinary cameras using dry cells or the like. To overcome these, it has already been proposed to obtain a long time exposure by providing single-frame photography means having a member for stopping the shutter in its closed position and a member for stopping the shutter in its open position, and by using such means to stop the motor in open shutter position during single-frame photography.
Such means is disclosed in U.S. Pat. No. 3,721,492, for example. However, in the type of long time exposure shown in that patent, when the release button is depressed to terminate long time exposure of one frame, the shutter is closed and a subsequent frame of film is fed and immediately begins to be exposed.
This is inconvenient for long time exposure wherein long time intervals intervene between adjacent frames of film to be exposed in such a manner that 20-second exposure of a frame is followed by a 5-minute interval which in turn is followed by 20-second exposure of a subsequent frame and then by a 5-minute interval, as in the microscopic photography of cell division, crystal growth or the like. Further, because of the two shutter stop members provided at two different locations for stopping the shutter in its open and its closed position, respectively, an effort to stop the shutter during continuous photography by releasing the release button might possibly stop the shutter in its open position depending on the timing of the release, with an inconvenient result that a portion of the film to be still unexposed is prematurely exposed.